1. Field of the Invention
The present invention relates to a tennis racket frame made of fiber-reinforced resin and more particularly to the tennis racket frame which is improved in fiber-reinforcing materials and distribution amount thereof in throat and handle sections thereof and in a sectional configuration thereof so as to improve ball-rebounding performance for hitting a ball and vibration-absorbing performance for giving a pleasant feeling to a player, which are contradictory to each other.
2. Description of Related Art
In recent years, the tennis racket frame is made of fiber-reinforced resin because it gives freedom in designing. Normally, the fiber-reinforced resin is molded into the configuration of the tennis racket frame as follows: A plurality of prepreg sheets laminated one on the other around a core material is placed in the cavity of a mold in the configuration of the tennis racket frame and then heated.
Carbon fiber and glass fiber have been mainly used as the fiber-reinforcing material of the prepreg sheet. But the glass fiber is not shock-resistant and not light and the carbon fiber is not shock-resistant, either.
In order to allow the tennis racket to have a high function, proposals have been made in recent years to use aromatic polyamide fiber or all-aromatic polyester fiber having a high strength and a high elastic modulus as a fiber-reinforcing material in combination with the glass fiber or the carbon fiber.
In order to allow a player to feel pleasant in hitting a tennis ball, it is necessary that the tennis racket has ball-rebounding performance and vibration-absorbing performance and is light so that the player can swing it easily. Further, the tennis racket frame is required to be durable.
The aromatic polyamide fiber and all-aromatic polyester fiber have a high strength and a high elastic modulus and thus shock-resistance and have a superior vibration absorbing performance and are light as described above. Thus, these fibers can be preferably used as fiber-reinforcing materials for allowing the tennis racket to have the above-described benefits. Japanese Patent Laid-Open Publication No. 62-142572 disclosed that the vibration-absorbing effect of the racket frame can be improved by using the aromatic polyamide fiber as the material of an inner fiber layer thereof. Japanese Patent Laid-Open Publication No. 1-141678 disclosed the use of a layer comprising the all-aromatic polyester fiber hardened with resin as an intermediate layer of a shaft of the racket frame at a volume percentage of 3 to 15 in order to improve shock-resistant strength and flexibility thereof.
According to the above-described conventional racket frames made of fiber-reinforced resin comprising the aromatic polyamide fiber or the all-aromatic polyester fiber used as the fiber-reinforcing material, it cannot be said that the feature of the aromatic polyamide fiber or the all-aromatic polyester fiber is effectively utilized to enable the tennis racket to have the above-described advantages.
That is, the shock-absorbing performance of the racket frame and the ball-rebounding performance thereof are contradictory to each other. The aromatic polyamide fiber or the all-aromatic polyester fiber, which have high strength and elastic modulus, used as the material of the all inner layer of the racket frame or the all intermediate layer thereof is capable of improving the shock-absorbing performance while it degrades the ball-rebounding performance.
The ball-rebounding performance is closely related to the rigidity of the racket frame. That is, the ball-rebounding performance can be improved by increasing the rigidity of the throat section including the flake section and the handle section. In other words, if the rigidity of these sections is small, the racket is flexed to a great extent when the ball collides with the racket. As a result, the energy generated by the collision between the ball and the racket cannot be effectively utilized.
On the other hand, if the rigidity of the entire frame is large, the vibration-absorbing performance degrades.
The ball-rebounding performance of the racket frame can be improved and the vibration-absorbing performance thereof is not deteriorated only in case of making large the rigidity of the throat section including the flake section and that of the handle section. It is known that the rigidity of the racket frame is determined by the material composing the racket frame and the sectional configuration thereof.
The greater the rigidity is, the higher the strength is. Accordingly, the strength of the throat section including the flake section and the handle section can be improved by making the rigidity thereof large, namely, by improving the ball-rebounding performance thereof. As a result, the durability of the racket frame can be improved.